This invention relates to demodulation of phase/frequency-modulated data generally and more particularly to zero intermediate frequency receivers.
Zero intermediate frequency (zero IF) receivers, also referred to as xe2x80x9cdirect conversionxe2x80x9d receivers typically employ a local oscillator operating at the carrier frequency of the received RF signal, coupled to a pair of quadrature related mixers to mix down the received RF signal to two quadrature related signals (I and Q), which may the be decoded or demodulated into a digital data stream. Such receivers are often employed as digital paging receivers or transceivers using an FSK (Frequency Shift Keying) modulation scheme, and may also be employed as cordless telephone transceivers. Recently, as described in U.S. patent application Ser. No. 09/302,932 for a xe2x80x9cTelemetry System for Implantable Medical Devicesxe2x80x9d, filed Apr. 30, 1999 by Villaseca et al, incorporated herein by reference in its entirety, such transceivers may also be employed as telemetry transceivers in implantable medical devices such as pacemakers, defibrillators and the like.
Typically zero IF receivers either provide phase shifted versions of the received signal to the mixers, where the signals are mixed with the local oscillator signal to provide I and Q output signals which are 90 degrees out of phase with one another or provide the received signal in phase to both of the two mixers, where the received signals are mixed with phase shifted versions of the local oscillator signal to provide corresponding I and Q output signals. Examples of these two basic circuit geometries are illustrated in U.S. Pat. No. 5,584,068 and U.S. Pat. No. 4,993,048. While receivers of these types have proven themselves to be extremely useful over a wide range of applications, AM (Amplitude Modulation) rejection remains a significant weakness. In particular, in such receivers employing balanced mixers, an imbalance in the mixer diodes or a DC offset between the mixers can lead to AM detection.
The present invention is intended to provide a zero IF receiver having improved AM rejection capabilities. The invention accomplishes this result by means of a receiver circuit, which employs three mixers, rather than the traditional pair of mixers as described above. The three mixers are preferably all located on the same integrated circuit chip. The received signal is input in phase to all three mixers where the received signal are mixed down with local oscillator signals phase shifted by 0xc2x0, xe2x88x9290xc2x0, and 180xc2x0 to provide three phase shifted mixer output signals at the baseband frequency. The mixer output signals are provided to two summing circuits, which take differences between the signals to produce I, and Q baseband frequency signals shifted 90xc2x0 relative to one another. Because the received signals are provided to the mixers in phase, assuming the mixers are on the same chip and thus nearly identical, the AM detection by each mixer should correspondingly be nearly identical. As a result, when differences between the signals are taken in the summing circuits, the common AM signals should nearly cancel, improving AM rejection and increasing the linearity of the mixer circuit.